Available online at www.sciencedirect.com
Energy Procedia 37 (2013) 485 – 493
GHGT-11
Amino acid salts for CO2 capture at flue gas temperaatures. Steven Chiao-Chien Wei*, Graeme Puxty and Paul Feron CSIRO Energy Technology, 10 Murryy Dwyer Circuit, Steel River Estate, Mayfield West, NSW 2300, Ausstralia
Abstract An amino acid salt, potassium taurate has been chosen as a high temperature absorbent in this studyy due to its low volatility and high absorption rate. The deensities and viscosities of 2M-6M taurate solution have beeen determined over the temperature range from 293K to 353K. 3 The CO2 solubility of taurate solutions has been meeasured using a stirred-cell reactor. It has been found that t the CO2 solubility of taurate solutions is comparabble to that of alkanolamines at high temperature. The absorption rate of CO2 into CO2 free and CO2 loaded taurate solutions were obtained using a wetted-wall column. The KG of 4M taurate at 353K is similar in magnitude to the KG of 5M MEA at 313K. It has also been found that the KG of o taurate decreased with increased CO2 loading, but the vaalues for KG of taurate solutions are still comparable to CO O2 loaded 5M MEA solution. © 2013 2013 The byby Elsevier r Ltd. © TheAuthors. Authors.Published Published Elsevier Ltd. Selection and/or peer-review under responssibility of GHGT Selection and/or peer-review under responsibility of GHGT Keywords:Absorbent; Taurine; Wetted-Wall Column; VLE; CO2 solubiity;Overall mass transfer coefficient.
1. Introduction A general consensus from the work of many climate scientists has emerged indicatingg that global warming and climate change are the result of the anthropogenic emissions of greenhousse gases. It is believed that carbon dioxide is the moost important anthropogenic greenhouse gas. The maajor source of anthropogenic carbon dioxide is the coombustion of fossil fuels (coal, natural gas and oil) whhich currently supply over 85% of world energy use. Of O this energy, approximately 40% is produced in pow wer plants. [1, 2] The technology of post combustion capture (PCC) is well recognized by government andd industry as a C 2 emissions from fossil fuel-fired power plants. [3] The captured way to effectively absorb 80-90% of CO CO2 can be stored in depleted oil annd gas fields, deep saline aquifers and unmineable coal c seams to reduce the CO2 emissions to the atmospphere from power plants. Most commercially available PCC processes p use liquid absorbents such as aqueous ammonia or alkanolamine solutions which typical absorb CO2 at temperatures between 283K and 3113K. [4] The a obviously temperatures of flue gases in fossil fueel fired power plants range from 393K to 433K and are
* Corresponding author. Tel.: +61-2-496062770; fax: +61-2-49606021. E-mail address:
[email protected].
1876-6102 © 2013 The Authors. Published by Elsevier Ltd.
Selection and/or peer-review under responsibility of GHGT doi:10.1016/j.egypro.2013.05.134
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higher than the temperature of absorption in PCC processes. Therefore, additional cooling systems and equipment are required to cool flue gases down to lower temperatures for the PCC processes. This cooling load adds to the cost, water consumption and energy consumption of PCC processes. Peeters et al. have evaluated the capital cost and energy penalty for the additional cooling system and pumps in a natural gas-fired power station. The contributions to capital cost and energy penalty are 3% and 10%, respectively. [5] Fisher et al. have also found the cost of the additional cooling system and pumps in a coal-fired power station is about 4% of total equipment cost. The energy penalty to maintain the cooling system is about 10% of total energy consumption. [6] Clearly, absorbents which can absorb CO2 above 313K and as close to flue gas temperature as possible, constitute a technological breakthrough with a high potential for a more economic and energy efficient capture process. In addition, the mass transfer between flue gases and absorbents could be faster at high temperature due to the lower viscosity and faster reaction kinetics under high temperature conditions. Most commercial absorbents are amine-based solvents. The solvents are not suitable to absorb CO2 at high temperatures (>333K) due to absorbent volatility and thermal degradation. [7] Recently, researchers have found that functionalized ionic liquids (ILs) can maintain their CO2 solubility at high temperature. Wang et al. [8] have found the mixture of IL and superbases can absorb CO2 at temperature up to 353K. A new type of polyamine based IL can absorb CO2 at temperatures between 383 and 403K. [9] However, to reach the maximum CO2 capacity in the ionic liquids, it requires at least 30 minutes of contact time between gas and liquid. The absorption rate of the ionic liquids is much slower than amine solutions. In addition, the cost of ILs is currently much greater than amine solutions due to the complex synthesis procedure. These issues mean that in the near term ILs might not be practically useful as an absorbent. Amino acid salts have been widely used as absorbents in the field of CO2 capture due to their low volatility and resistance to oxygen degradation in the absorption process. [10-15] Van Holst et al. have investigated the apparent rate constants for several amino acid salts at 298K to find suitable absorbents for CO2 capture. They found the amino acid solutions such as glycinate, prolinate, sarcosinate and taurate exhibit relatively high reaction rate constants as compared to MEA solution. [12] Kumar et al. have measured the solubility of CO2 in taurate solution at 298K and 313K for a range of CO2 partial pressure from 0.1 to 6kPa. The kinetics of CO2 absorption in taurate solution has also been investigated over a range of temperatures between 285K and 305K by using a stirred cell reactor. [11,15] However, their studies have only investigated the properties of amino acid salts at temperatures ranging from 283K to 333K. So far, there are very few studies using amino acid salts to absorb CO2 at high temperatures. It has been noticed that higher energy consumption can be required for CO2 capture at high temperature. Although CO2 absorption at high temperatures could result in the increase of reboiler heat duty, the capital cost of flue gases cooling systems and the saving from in terms of space for the system also need to be considered. This study focused on the performance of CO2 capture at temperatures between 323K and 373K by using potassium taurate solutions (2M ~ 6M). Taurate solutions were chosen due to their relatively low cost, low volatility and thermal stability at high temperature. The physical properties of taurate solutions such as density and viscosity have also been measured in the study. 2. Methodology 2.1 Density and Viscosity measurements Taurine ( 90%) and potassium hydroxide ( 90%) were purchased from Sigma-Aldrich and were used directly without further purification. Densities of unloaded and loaded taurate solutions were measured using a density meter (DMA 38, Anton Paar) at temperatures between 298K and 313K with an error of ±0.001 g cm-3. A traditional method measuring the mass and the volume of the solution was used to determine the densities of the taurate solutions at temperature above 313K due to the limit of the density
Steven Chiao-Chien Wei et al. / Energy Procedia 37 (2013) 485 – 493
meter. The traditional method was carried out by filling the taurate solutions in a volumetric flask (Pyrex) and weighting the mass of the solution using a balance (GR 300, A&D Weighing). The volume of the volumetric flask had been calibrated at temperatures from 313K to 353K using deionized water. The expanded volume at different temperatures was calculated by using the mass of water in the flask divided by the density of water obtained from Perry’s handbook [16] Dynamic viscosities of the taurate solutions were measured using a viscometer (AMVn, Anton Paar) at temperatures between 298K and 353K with a specified repeatability
